The present invention relates broadly to a method and apparatus for measuring the ohmic resistance at the area of contact between body tissue and an electrode attached to the tissue. By way of illustration, the invention is directed to a method and apparatus for measuring the ohmic contact resistance between cardiac tissue and a pacemaker electrode so that the electrode can be surgically implanted in the heart at a location where the contact resistance is relatively low, thereby assuring more reliable operation of the pacemaker.
The natural atrial-ventricular stimulating system of the heart in many adults and some children is defective. In some cases, a defective natural stimulating system can be aided by a pacemaker of either the external or implantable type. A pacemaker includes miniaturized, battery-powered, self-contained pulse generator for generating pulses and an electrode for conducting the pulses to the heart. Most pacemaker pulse generators generate pulses having a fixed amplitude although the amplitude of pulses generated by some pacemaker pulse generators is adjustable. The pulses provide pacing signals for periodically stimulating the heart, thereby causing the heart to beat in the way a normal natural stimulating system causes the heart to beat. The frequency of the pulses generated by the pulse generator is preadjusted so that the pulses cause the heart to beat at a desired rate.
One known procedure for installing or replacing a pacemaker is merely to surgically implant the pacemaker electrode on as trial-and-error basis until a location is found where the pulses generated by the pacemaker pulse generator cause the heart to beat at the desired rate. Such a procedure overlooks the possibility that the resistance at the area of contact between cardiac tissue and the electrode may be relatively high at the time the electrode is implanted which in turn can mean that the initial threshold for stimulating the heart may be relatively high. The contact resistance can reasonably be expected to increase due to formation of scar tissue at the point where the electrode is implanted as the wound heals. As a result, the threshold for stimulating the heart may become even higher. Therefore, even though a pacemaker having a fixed amplitude pulse generator causes the heart to beat at the desired rate at the time the electrode is implanted, the increase in contact resistance caused by formation of the scar tissue as the wound heals may cause the threshold for stimulating the heart to become so high that the pulses generated by the pulse generator no longer cause the heart to beat. In pacemakers having adjustable amplitude pulse generators, if the initial threshold for stimulating the heart is relatively high due to relatively high resistance at the area of contact between cardiac tissue and the electrode, the amplitude of the pulses generated by the pulse generator must be adjusted upwardly until the pulses cause the heart to beat at the desired rate. However, the high voltage needed to cause the heart to beat may precipitate deterioration of cardiac tissue around the electrode which means that additional scar tissue will form even after the wound at the point where the electrode is implanted heals which in turn means that the contact resistance can reasonably be expected to increase. As a result, the threshold for stimulating the heart may become so high that the pulses generated by the pulse generator no longer cause the heart to beat.
Furthermore, high contact resistance causes greater attenuation of the natural stimulus fed back for inhibiting the pacemaker pulse generator in a demand-type pacemaker. Consequently, the pulse generator may generate a pulse even though the natural stimulating system causes the heart to beat. As a result, the pulse from the pulse generator presents the risk of inducing fibrillation which can cause death.
A more qualitative procedure for installing or replacing a pacemaker is discussed by J. W. Calvin, "Intraoperative Pacemaker Electrical Testing," Ann. Thorac. Surg., Volume 26, Page 165 (1978). Calvin calculates an impedance by dividing the known pulse generator voltage by the measured current through the electrode after the threshold is set, and he reports that an acceptable range for impedance is 300-800 ohms. S. S. Barold and J. A. Winner, "Techniques and Significance of Threshold Measurements for Cardiac Pacing," Chest, Volume 70, Page 760 (1976), calculate the impedance by dividing the average pulse generator voltage measured on an oscilloscope by the average current through the electrode also measured on an oscilloscope. Those authors also indicate that some commercially available threshold analyzers measure the pulse generator voltage and the current through the electrode needed for calculating the impedance at an arbitrary point in time, for example, 90 microseconds, from the onset of a pulse.
Merely calculating the impedance, however, has certain drawbacks since the calculated impedance does not indicate the state of the conductive pathway which determines whether or not the pulses generated by the pulse generator cause the heart to beat. The state of the conductive pathway is determined mainly by the resistance at the area of contact between cardiac tissue and the electrode. Specifically, the threshold for stimulating the heart increases linearly with an increase in the contact resistance. Consequently, the contact resistance is the critical factor in determining whether or not the pulses generated by the pulse generator cause the heart to beat. Also, if the contact resistance is high, the natural stimulus fed back for inhibiting the pulse generator in a demand-type pacemaker may be attenuated to such an extent that the pulse generator will not be inhibited resulting in the risk of inducing fibrillation which can cause death. Therefore, the separate value of contact resistance is important in determining whether or not the location of the electrode should be changed.